Photonic integration is considered to be a key technology for future advancement in optical communication technology. Scaling down of the optical building blocks enables cost-effective, complex and ultra-compact photonic circuits, i.e. chips which comprise integrated optical components formed on or in a substrate and which are interconnected by planar waveguides. It is typically desirable to be able to optically couple the signals conducted by such planar waveguides into or out of the integrated chip, for example from or into an optical fiber.
Choosing an appropriate platform for developing this miniaturization technology is guided by functionality, compatibility, performance, yield and cost. Currently, silicon-on-insulator (SOI) can be considered as the leading technology for low-cost and high-volume photonic integration, since it benefits from processes developed in the mature electronics industry. Although the high refractive index contrast of SOI reduces the footprint of integrated photonic devices considerably, it becomes more difficult to achieve a high-performance mode-size convertor between a single-mode optical fiber, with a typical cross-sectional area of 100 μm2, and an on-chip integrated optical waveguide, with a typical cross-sectional area in the order of 0.1 μm2.
A possible solution is a grating coupler which is a periodic structure that couples light out of the chip, e.g. to free space or to an optical fiber. Fiber-to-chip grating couplers with very high efficiency have been demonstrated. However, a typical reflection back into the on-chip waveguide for such a high-efficiency grating coupler may be around −17 dB, while high back-reflections of −10 dB and −8 dB have been reported as well. These levels of back-reflection may be unacceptable for integrated circuits, especially for example for circuits which contain integrated lasers, or which implement interferometer-based designs.
For grating couplers, two main sources may contribute to the back-reflection, i.e. reflection back into the waveguide. The second order reflection of the grating may be considered the dominant source. Typically this second order reflection may be eliminated by tilting the optical fiber under a small angle of around 10° with respect to the surface normal. The second source of back reflection is due the Fresnel reflection at the grating coupler interface. This reflection may be highly dependent on the grating structure itself and may therefore be very difficult to eliminate.
Besides optimizing reflection and coupling efficiency, there is also an incentive for further reducing integrated photonics circuits in size. In “compact focusing grating couplers for Silicon-on-Insulator integrated circuits” by F. Van Laere et al. in IEEE Photonics Technology Letters 19(23) 2007 p 1919-1921, a compact focusing grating coupler is presented which uses a curved grating to focus the coupled light onto a single-mode waveguide, achieving an eight-fold length reduction as compared to a conventional linear grating with adiabatic taper, without performance penalty. More specifically, this grating is elliptically curved, i.e. comprises a plurality of grating ridges having an elliptical profile. The radiation is coupled to the grating coupler along the direction of the long axes of these grating ridges, and due to the curvature of the grating, the distance between the grating and the output opening of the waveguide can be drastically reduced.
In U.S. Pat. No. 7,184,627 B1, a grating coupler is disclosed, which comprises a plurality of scattering elements, and at least one distributed Bragg reflector to reflect radiation passing through the grating towards the substrate of the grating coupler back toward the grating.